This invention relates to an improved exhaust gas oxygen sensor of the type adapted for installation in a conduit for conveying exhaust gases from an internal combustion engine. The improved sensor is responsive to the partial pressure of oxygen in the exhaust gases to which the sensor is exposed and has an electrical characteristic which varies, when the sensor is at operating temperatures in the range from about 350.degree. C. to about 850.degree. C., with the partial pressure of oxygen in the exhaust gases.
Exhaust gas sensors have been fabricated using either a ceramic zirconia tube or a titania oxygen sensing element of disc shape, the latter having electrode wires in embedded in the titania disc. Both sensor types convey to an electronic control system information concerning variations in an electrical characteristic of the oxygen sensing element as the composition of the exhaust gases from an internal combustion engine is varied. In the case of titania sensor, electrical lead wires and the titania oxygen sensing element are supported by a ceramic member mounted within a steel body that in use is attached to the exhaust conduit of an engine. The sensor is subjected to exhaust gases of varying composition, and a consequentially varying electrical signal passes from the sensing element to terminal pins at the end of the ceramic member. It has been found necessary with the exhaust gas sensor of this design to provide a seal between the steel body and the ceramic member and also to provide a perforated protection tube that surrounds the portion of the ceramic member designed to project into the exhaust conduit.
Protection tubes for exhaust gas oxygen sensor devices are known, as may be seen for example in commonly assigned U.S. Pat. No. 4,001,758 issued Jan. 4, 1977 to M. J. Esper, W. L. Green, S. R. Merchant and C. M. Wells, one of the present inventors. The above-mentioned patent relates to a titania exhaust gas sensor. Other sensor types are known as is evidenced by the following U.S. Pat. Nos. issued to Robert Bosch GmbH of Stuttgart, Germany: 3,891,529 issued June 24, 1975 to O. Beesch; 3,841,987 issued Oct. 15, 1974 to K. H. Friese et al; and 3,978,006 issued Aug. 31, 1976 to B. Topp et al. These patents of Robert Bosch GmbH illustrate various designs and arrangements for exhaust gas sensors of the zirconia type. U.S. Pat. No. 3,891,529 illustrates a protection tube used in connection with a zirconia ceramic member and the sealing arrangement achieved between the protection tube and the zirconia member, which in this type of device is responsive to the partial pressure of oxygen. It should be noted that the zirconia sensors apply a tubular element which must be sealed on one side from the exhaust gases which on the other side must be subjected thereto. The side of the zirconia sealed from the exhaust gases is supplied with a reference gas, usually with air, and the exhaust sensor output electrical signal is obtained as an EMF generated across platinum electrode material applied on opposite sides of the zirconia tube to hold a platinum surface on its exterior in electrical contact with the steel body or with the protection tube surrounding the zirconia tube. Spring pressure may be used for this purpose.